By judiciously modulating the ring strain and sterics, we developed a class of disubstituted cyclopropenes that selectively underwent single monomer addition in ring-opening metathesis but readily underwent alternating ring-opening metathesis polymerization with low-strain cyclic olefins in a living fashion. The substituents on cyclopropenes effectively inhibited homoaddition and prevented secondary metathesis on the polymer backbone. The resulting polymers had controllable molecular weights and end groups, very low dispersities, and high regularity in microstructure under optimized conditions. (1)H and (13)C NMR spectroscopy and MALDI-TOF MS showed a rigorously alternating sequence. Interestingly, disubstituted cyclopropenes were found to present zero-order kinetics, indicating their rapid single addition and the rate-determining ring opening of the low-strain olefin.